Injection lasers include a wide variety of lasers. Most of these are of the Gallium-Arsenide family and are currently not used for spectroscopic purposes. There are also known other injection lasers, such as lasers of the lead-salt family, including (Pb,Sn)Te, which are currently used for spectroscopic applications. Lasers of this type are commonly termed, tunable diode lasers.
It is well known that the resonance frequency of tunable diode lasers is dependent on the temperature of the active region of the laser. The maintenance of a predetermined resonance frequency to high precision over time is of significance when the laser is being used for spectroscopic examinations.
Laser spectroscopes are known for performing very high accuracy spectroscopic measurements to the order of 10.sup.-4 cm.sup.-1. These systms which are commercially available from Laser Analytics, of Bedford, Mass., U.S.A., normally comprise a laser which radiates through a sample gas onto a detector, which provides an output indication of the absorption of the laser radiation at the precise radiation frequency.
The accuracy of these measurements is limited by the temperature stability of the laser head which is usually maintained by using a temperature sensor such as a silicon diode and appropriate electronic apparatus.
In order to overcome the problem of frequency stabilization, a current feedback arrangement has been used by a number of investigators. See I. I. Zasavitskii et al, "Temperature Regulation Of A Tunable Diode Laser Within (about) 10 -5 K", Soviet Tech. Phys. Lett. Vol 8(10), October, 1982 pp. 502, 503. In this arrangement, the laser beam is split into two branches, one of which passes through a measurement cell and one of which passes through a cell containing a reference gas and impinges on a detector which measures the absorbed radiation.
An absorption line shape of the reference gas is obtained directly by modulating the electric current operating the laser, thus modulating the laser radiation frequency around the central frequency of the absorption line. This line shape is detected by special apparatus which operative to control the bias current of the laser via a current feedback mechanism. Thus any line shift evolving in the system as a result of a temperature fluctuation induces a current of opposite effect which pushes the line center back to its predetermined place. Thus the laser central frequency is locked to the center of the absorption line.
There are some significant disadvantages to the current feedback approach to laser frequency stabilization in which temperature fluctuations are compensated by current fluctuations, since current fluctuations induce inherent radiation intensity instability in the laser. Furthermore, the laser radiation is modulated at a whole predetermined range around the frequency of the center absorption line, but only this center frequency is stabilized while the side band radiation can still fluctuate in both frequency and intensity according to the current/frequency/temperature interrelationship.
There has also been proposed by H. Tsuchida et al in an article entitled "Frequency Stability Measurement of Feedback Stabilized AIGaAs DH Laser" in Journal of Applied Physics, Vol. 19, No. 12, December, 1980, pp. 1721-1724, a technique whereby a thermal module associated with the laser is controlled by an output signal produced by a feedback arrangement incorporating a Fabry-Perot interferometer.
In this arrangement, the laser operates under conditions of constant current and constant temperature, thus eliminating the disadvantages of current feedback described hereinabove. However, the Fabry-Perot fringes which are used for locking the radiation frequency of the laser are much broader than absorption lines of gases in most cases. Therefore, the degree of stabilization that can be achieved with this apparatus is lower than that which can be achieved using gas absorption lines as the active feedback parameters. Furthermore, the Fabry-Perot interferometer is itself sensitive to temperature and atmospheric pressure in its immediate environment thus adding additional instability factors.